1. Technical Field
The present disclosure relates generally to a wireless charging method for a vehicle, and more particularly, to a method and an apparatus using a variable switching frequency in order to obtain a target output voltage.
2. Related Art
Due to environmental pollution and oil energy depletion, world-wide studies on environment-friendly electric vehicles (EV) have been ongoing. As demands and developments relating to EVs and plug-in hybrid vehicles (PHEV) increase, the on-board charger (OBC) for high-voltage battery charging becomes an essential component in the automobile industry.
Meanwhile, instead of conductive charging for which connectors are used, wireless power transfer (WPT) technologies used for charging high-voltage batteries without connectors have been introduced. WPT technologies have made rapid progress in application domains such as portable device charging and electronic vehicle charging. Notably, wireless charging technologies providing battery charging and freedom from short circuits and wire disconnections have been introduced in the electronic vehicle charging domain.
In a wireless charging system for EV charging, a primary pad and a secondary pad can be modeled as a transformer in an equivalent circuit. As compared with conventional convertor transformers, a coupling coefficient is relatively low since an air-gap between the primary pad and the secondary pad is typically large. That is, since magnetizing inductance is much larger than leakage inductance, it may become difficult to transfer power to an output.
Therefore, a method in which at least one capacitor is applied to the primary pad and secondary pad so that a resonance between the pads is caused by inductance and capacitance of the pads is commonly used. However, when a misalignment between the primary pad and the secondary pad exists, the magnetizing inductance and the leakage inductance may vary, and consequently, the resonance between the pads degrades, and the output power and efficiency of the power transfer decrease. Also, due to manufacturing tolerances of the primary pad, secondary pad, and capacitor, the resonance between the pads may degrade so that the output power and efficiency of the power transfer decrease.